Infections of the nail and hoof, known as ungual and/or periungual infections, pose serious problems in dermatology. These ungual and/or periungual can be caused by sources such as fungi, viruses, yeast, bacteria and parasites. Onychomycosis is an example of these serious ungual and/or periungual infections and is caused by at least one fungus. Current treatment for ungual and/or periungual infections generally falls into three categories: systemic administration of medicine; surgical removal of all or part of the nail or hoof followed by topical treatment of the exposed tissue; or topical application of conventional creams, lotions, gels or solutions, frequently including the use of bandages to keep these dosage forms in place on the nail or hoof. All of these approaches have major drawbacks. The following discussion is particularly directed to drawbacks associated with current treatment of ungual and/or periungual antifungal infections.
Long term systemic (oral) administration of an antifungal agent for the treatment of onychomycosis is often required to produce a therapeutic effect in the nail bed. For example, oral treatment with the antifungal compound terbinafine typically requires administration of 200 to 400 mg/day for 12 weeks before any significant therapeutic benefit is realized. Such long term, high dose systemic therapy can have significant adverse effects. For example, terbinafine has been reported to have liver toxicity effects and reduces testosterone levels in blood due to adverse effects on the testes. Patient compliance is a problem with such long term therapies especially those which involve serious adverse effects. Moreover, this type of long term oral therapy is inconvenient in the treatment of a horse or other ruminants afflicted with fungal infections of the hoof. Accordingly, the risks associated with parenteral treatments generate significant disincentive against their use and considerable patient non-compliance.
Surgical removal of all or part of the nail followed by topical treatment also has severe drawbacks. The pain and discomfort associated with the surgery and the undesirable cosmetic appearance of the nail or nail bed represent significant problems, particularly for patients more sensitive to physical appearance. Generally, this type of treatment is not realistic for ruminants such as horses.
Topical therapy has significant problems too. Topical dosage forms such as creams, lotions, gels etc., can not keep the drug in intimate contact with the infected area for therapeutically effective periods of time. Bandages have been used to hold drug reservoirs in place in an attempt to enhance absorption of the pharmaceutical agent. However the bandages are thick, awkward, troublesome and generally lead to poor patient compliance.
Hydrophilic and hydrophobic film forming topical antifungal solutions have also been developed. These dosage forms provide improved contact between the drug and the nail. Topical formulations for fungal infection treatment have largely tried to deliver the drug to the target site (an infected nail bed) by diffusion across or through the nail.
Nail is more like hair than stratum corneum with respect to chemical composition and permeability. Nitrogen is the major component of the nail attesting to the nail's proteinaceous nature. The total lipid content of mature nail is 0.1-1.0%, while the stratum corneum lipid is about 10% w/w. The nail is 100-200 times thicker than the stratum corneum and has a very high affinity and capacity for binding and retaining antifungal drugs. Consequently little if any drug penetrates through the nail to reach the target site. Because of these reasons topical therapy for fungal infections have generally been ineffective.
Compounds known as penetration or permeation enhancers are well known in the art to produce an increase in the permeability of skin or other body membranes to a pharmacologically active agent. The increased permeability allows an increase in the rate at which the drug permeates through the skin and enters the blood stream. Penetration enhancers have been successful in overcoming the impermeability of pharmaceutical agents through the skin. However, the thin stratum corneum layer of the skin, which is about 10 to 15 cells thick and is formed naturally by cells migrating toward the skin surface from the basal layer, has been easier to penetrate than nails. Moreover, known penetration enhancers have not proven to be useful in facilitating drug migration through the nail tissue.
Antimicrobial compositions for controlling bacterial and fungal infections comprising a metal chelate of 8-hydroxyquinoline and an alkyl benzene sulfonic acid have been shown to be efficacious due to the increased ability of the oleophilic group to penetrate the lipoid layers of micro-cells. The compounds however, do not effectively increase the ability to carry the pharmaceutically active antifungal through the cornified layer or stratum corneum of the skin. U.S. Pat. No. 4,602,011, West et al., Jul. 22, 1986; U.S. Pat. No. 4,766,113, West et al., Aug. 23, 1988.
Therefore, there is a need in the art for compounds which can effectively penetrate the nail. There is also need in the art for compounds which can effectively treat ungual and/or periungual infections. These and other needs are addressed by the current invention.
Aminoacyl-tRNA synthetases (ARS) are a family of essential enzymes that attach amino acids to the 3′ terminal adenosine end of tRNAs, the charged tRNAs are then used by the translation machinery to synthesis proteins from mRNA. Although there are few exceptions, for example in Gram-positive bacteria and archaea, most organisms have at least one ARS for each amino acid. In the case of eukaryotes, they have two ARS, one is localized to the cytoplasm while the other ARS is located in the organelle(s). The ARS catalyzes two reactions, as outlined below, the first reaction adenylates the amino acid with ATP followed by its transfer to the 2′- or 3′-hydroxyl of the terminal adenosine of tRNA.                Amino acid (AA)+ATP→AA−AMP+PPi;        AA-AMP+tRNA→tRNA−AA+AMP        
The family of 20 ARS fall into two distinct structural classes as determined by their crystal structure. Class I, which have a Rossman like fold, include the ARS for the following amino acids-arginine, cysteine, glutamate, glutamine, isolelucine, leucine, lysine (in archaea and some bacteria), valine, methionine, tryptophan and tyrosine. Class II ARS include the enzymes for the amino acids, alanine, asparagine, aspartate, glycine, histidine, lysine, phenylalanine, proline, serine and threonine. The ARS mediated reaction is the major checkpoint of specificity that ensures the correct amino acid is charged to its cognate tRNA. Since some amino acids only differ by a single methylene group, for example valine and isoleucine, it has been postulated that the specificity of the synthetic reaction alone can't explain the observed in vivo accuracy of tRNA charging. The synthetic active site should be able to exclude amino acids that are not close analogs of the cognate amino acid, but analogous amino acids pose a bigger problem. Therefore to increase specificity, proof-reading and editing must occur. So far nine ARS have been shown to have an editing mechanism that significantly reduces the frequency of mischarged tRNAs. The enzymes for the following amino acids have been shown to have editing activity-alanine, isoleucine, leucine, methionine, lysine, phenylalanine, proline, threonine and valine. These ARS can hydrolyse the incorrectly adenylated amino acid AA-AMP (pre-transfer editing) or the incorrectly charged tRNA (post-transfer editing). To date the isoleucyl, leucyl and valyl-tRNA synthetases have the best-characterized editing mechanisms; an additional structural domain called the connective polypeptide I (CP1) inserted in the synthetic domain has been shown to contain the editing active site. This is located more than 25 Å away from the synthetic active site, which suggests that both the adenylated amino acid intermediate and amino acid tethered to the 3′ end of the tRNA must be moved from the active site in the synthetic domain to the editing site for the reaction to be proof-read. It has been postulated that the 3′ end of the charged tRNA is translocated in a similar manner to that of the proof-reading mechanism of DNA polymerases. Much less is known about the translocation of the adenylated amino acid. A similar CP1 domain is also present in the methionine and cysteine ARS enzymes, but it is much smaller than that found in the valine, isoleucine and leucine enzymes. Despite the absence of a direct homolog to the CP1-like domain in class II ARS, separate editing domains have been found in the enzymes for proline and threonine. Although editing is important to ensure the correct charging of tRNAs, it is not essential for viability and is not required for the synthesis of charged tRNAs. For example, in Escherichia coli, in which 10 amino acids in the editing domain of isoleucyl-tRNA synthetase were changed to alanine, the resulting mutant was still viable, although it did have many pleiotropic effects, including a noticeable cell growth defect.
In spite of significant homologies between human, bacterial and fungal ARS there are a number of compounds that have been developed as anti-infectives. The most notable example of an ARS inhibitor is the commercial antibiotic mupirocin (pseudomonic acid), which is sold under the label Bactroban. Mupirocin specifically inhibits bacterial isoleucyl-tRNA synthetases, while its activity against the human homolog is more than 1,000 times less active. Mupirocin binds specifically to the synthetic active site and mutants that are resistant to this drug have mutations in the synthetic domain of leucyl-tRNA synthetase. Likewise, reveromycin A inhibits the eukaryotic isoleucyl-tRNA synthetases: Saccharomyces cerevisiae resistance mutants have mutations in the synthetic domain. So far all attempts to develop better ARS inhibitors than mupirocin, an isoleucine-adenylate analogue, have relied on inhibiting the synthetic reactions.
Since it has been previously thought not to be essential for the synthesis of charged tRNAs, the editing domain of tRNA synthetases has not been thought a promising target for drug development. Data from mutational analysis of the ARS editing domains tend to suggest that inhibition of the editing mechanism leads only to an increase in mischarged tRNAs and does not lead to cell death. Compounds that are active against, and specific for, the editing domain of the tRNA synthetase would provide access to a new class of antimicrobial therapeutics to augment the arsenal of agents currently in use. Quite surprisingly, the present invention provides such compounds and methods of using these compounds.